A question about memory usage

Hi all,

I have a server with 4GB of RAM and I wanted to know how much memory
is being used by a PostgreSQL. To do so I have executed the following:

ps -A -o rss,vsz,command|grep postgres | awk '{rss += $1; vsz += $2 }
END { print "Real: ",rss/1024"MB Virtual: ",vsz/1024"MB" }'

And the result is:
Real: 157.59MB Virtual: 6359.04MB

So I understand that amount of virtual memory must be swapped to disk as
it's bigger that my physical memory. To check this I have executed the
free command and I get the following results:

total used free shared buffers cached
Mem: 4153180 3988536 164644 0 161400 3117900
-/+ buffers/cache: 709236 3443944
Swap: 6215672 64 6215608

So my question is, what I'm doing wrong? because the swap it's almost
empty

Thank you very much
--
Arnau